A radio access network, RAN, is the part of a mobile telecommunication system that is responsible for connecting User Equipment, UEs, to the core network. The radio access network implements a certain radio access technology, such as third generation, 3G, fourth generation, 4G, and Long Term Evolution, LTE.
Different types of radio access network exist. For example, the Universal Terrestrial Radio Access Network, UTRAN, is a term used for the network and equipment that connect UE's to the public internet. The UTRAN comprises base stations which are referred to as Node B′s and Radio network controllers. Another example is the Evolved Universal Terrestrial Radio Access Network, E-UTRAN, which is initially meant as a replacement of the Universal Mobile Telecommunications System, UMTS. An Evolved Node B is the element in the E-UTRAN that is the evolution of the Node B in the UTRAN access network.
A random access procedure is typically used by User Equipment, UEs, to initiate a data transfer. Such a random access procedure is used to synchronize the UE towards the base station, i.e. from the UE towards an access node of the radio access network, i.e. the eNodeB or the node B.
When performing random access in the Long Term Evolution, a UE sends a specific pattern or signature called RACH preamble sequence. The sequence is derived from a root sequence and some transform that is broadcasted by the radio base station. This creates a number of possible preamble sequences that can be used towards this base station.
The preamble sequence selected by the UE is used to differentiate UEs from each other that makes access at the same time. Collision handling is typically also present later on in the random access procedure. The selection of the preamble sequence can also be configured to make the UE to take the current estimated path loss and message size and coverage enhancements techniques into account when selecting a preamble.
Which preamble root sequences that should be used towards a specific radio base station is broadcast to the UE. For a handover, there can also be a dedicated random access enabled where the UE will use a specific preamble sequence that is not configured/used by any other UE at the same time. This is called contention free random access. To avoid that a neighbour base station uses the same set of root preamble sequences the preamble sequences are planned or automatically configured or calculated. If the preamble root sequences are unique between two base stations, then only the base station that matches the preamble sequence used by the UE for the first random access message will respond. The above is valid for LTE and, on a high level expected to be valid also for 5G or new Radio Access Technologies, RAT.
The use of unique preamble sequences between two neighbour base stations, i.e. these two base stations having different RACH configurations, will ensure that only the base station which the UE is intended to perform random access towards will respond. There will be a one-to-one mapping between random access request, from a certain UE, and random access reply from the base station which matches the preamble sequence. The preamble sequence is selected by the UE from the RACH configuration sent out by each base station, and typically the UE will choose the strongest one.
There is a drawback of the procedure as mentioned above in that it does not provide for robustness. That is, requests sent by the UE may get lost and are thus not received by a radio access node. In such a case, a lost request is sent again, by the UE. This time, the request is sent with a higher transmit power to make sure that the radio access node will receive the request, i.e. that the request will not get lost. This procedure is, however, cumbersome.